One of the more popular open source IM clients is Pidgin (formerly GAIM).  This client works with pretty much all chat services including the majors MSN, Google Talk, AIM, and Yahoo!  Pidgin works with Linux, Windows, BSD, and other Unix distributions.  If you're a real minimalist you can even get a text based version.  It is very customizable and there is a lot of open source plug ins for it as well.  Pidgin supports pictures, emoticons, and fonts.  Very customizable IM alternative!

Choose your favorite chat service.

Here is an example of the menu choices available.

You can resize your messaging window to your liking.

Get more information and download Pidgin.